Physical and chemical properties of some italian opals - Periodico di

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYand applied topics on Environment, Archeometry and Cultural Heritage

DOI: 10.2451/2012PM0006Periodico di Mineralogia (2012), 81, 1, 93-106

PERIODICO di MINERALOGIAestablished in 1930

Introduction

Although Italian opals feature gemologicalqualities lower than those from foreign countries,

some specimens are highly prized by gemologistsand collectors. In Italy, opals can be found atseveral sites; the most frequent variety is thecommon (without any play-of-color) opal, which

Physical and chemical properties of some italian opals

Franca Caucia1,*, Luigi Marinoni1, Valentina Bordoni1, Christian Ghisoli1 and Ilaria Adamo2

1 Dipartimento di Scienze della Terra, Università degli Studi di Pavia, via Ferrata 1, 27100 Pavia, Italy 2 Università degli Studi di Milano, Dipartimento di Scienze della Terra, Sezione di Mineralogia,

Via Botticelli 23, 20133 Milano, Italy*Corresponding author: [email protected]

Abstract

The physical and compositional properties of some opals from different parts of Italy havebeen investigated through several methodologies like optical analysis, specific gravity,refractive indices, XRPD, IR spectroscopy, LA-ICP-MS. The opals show different colors:white, brownish-white, yellowish-white, yellow-/greenish white and grey. Black and metallicinclusions, consisting of todorokite, are sometimes present. Play-of-color have not beenobserved but some opals show small iridescence zones; opals are inert to the long and shortwavelength UV radiation (366 - 254 nm) with the exception of one sample; alsophosphorescence is absent. Refractive index and specific gravity values are respectively 1.43to 1.44 and 2.07 to 2.33 g/cm3 consistent with data available in the literature. XRPD analyseshighlighted Italian opals are A, CT and C types, but most of them can be classified as opals-CT. IR spectroscopy data confirmed the opal classification. The most abundant impurities areMg (between 400 and 900 ppm), Fe (35-400 ppm), Ca (72-96 ppm) and Ni (20-65 ppm).Similarly to what observed in other opals worldwide, Fe appears to be the principal factorthat determines the brown to red to orange to yellow colors. Chromophore elements like V,Cr, Cu, Ti, Co are present in very low concentrations and probably do not influence thephysical properties of the Italian opals analyzed. Ni is present in more relevant amounts andis probably related to clay minerals. Mn is clearly detected only in two samples and is relatedto the presence of dendrites. The investigated Italian opals show a rather homogeneous traceelement composition which is in the range of worldwide opal’s composition, except for theirnoticeable amount in Ni.

Key words: Italian opals, hyalite, IR spectroscopy, trace elements, XRPD.

Caucia et al_periodico 18/04/12 12:29 Pagina 93

is an opaque variety colored by iron oxides andhydroxides or by jasper, chalcedony, clay andother minerals (Jerwis, 1873; Hintze, 1915;Fenoglio and Sanero 1943; Mariani and Scaini,1978). Even if accurate investigations on theirmodes of formation are lacking, Italian opals aregenerally considered of hydrothermal origin (seealso Ghiara et al., 1999; Cerri et al., 2001; Altaneret al., 2003). Other opal varieties present in Italyare the following (Jerwis, 1873; Hintze, 1915;Fenoglio and Sanero, 1943; Mariani and Scaini,1978): hydrophane (opaque and very porousvariety of opal that becomes translucent ortransparent when wet), hyalite (opal with a glassyand clear appearance and a globular surface, thatsometimes shows internal play of colors andnatural inclusions; it is usually found in volcanicrocks), geyserite (opaline silica usually occurringaround hot springs and geysers; botryoidalgeyserite is known as fiorite, from Santa Fiora-Amiata Mt. in Tuscany), xyloid opal (also knownas wood opal, opalized wood) and diatomite(sedimentary rock almost entirely made up ofsilica remains of diatoms). Unlike specimensfrom countries such as Australia, Mexico, Brazil,Peru, Ethiopia, the physical and chemicalcharacteristics of Italian opals have been poorlyinvestigated. These properties can be related tothe body color or the luminescence of the opal,and also to the process of formation and the areaof provenance. As described in Gaillou et al.(2008), the identification of the chemical andphysical properties of opals from a particulargeographic area is important for several reasons.For instance, in the gem market, stones comingfrom specific localities can be more valuable thanothers. In addition, in the archaeometricalinvestigation, knowledge of the geographic originis crucial to reconstruct the ancient trade routesof gem (Giuliani et al., 2000; Gaillou et al.,2008). In this work the physical, mineralogicaland chemical properties of opals from differentareas of Italy have been investigated throughseveral methodologies, such as optical analysis,

specific gravity measurement, refractive indices,X-ray powder diffraction (XRPD), infraredspectroscopy (IR), and laser ablation inductivelycoupled plasma mass spectrometry (LA-ICP-MS). The data obtained are discussed and alsocompared with those of opals from differenttypologies and from other parts of the world.

Provenance of the investigated samples

Twelve samples of opals from different Italiansites have been analyzed, mostly from thePiedmont region, but also from Sardinia andEmilia Romagna. In particular, the opalsinvestigated in this work come from thefollowing localities:

1) Baldissero Canavese in the Province ofTurin, region of Piedmont. The samples weregifted by the “AMI”, (Associazione Micro-Mineralogica Italiana).

2) Mount Calvo at Caselette, in the Province ofTurin, region of Piedmont. The samples weregifted by the “Museo Don Bosco” of Turin.

3) Mount Maniglia near Bellino, Province ofCuneo, region of Piedmont. The samples weregifted by the “Museo Don Bosco” of Turin.

4) Case Montanini, at Sant’Andrea Bagni(Medesano), Province of Parma, region ofEmilia-Romagna. The sample belonged to thepersonal collection of the first author.

5) Silius in the Province of Cagliari in Sardinia.The sample was offered by the “Museo DonBosco” of Turin. A comlete list of samples withtheir properties can be found in Table 1.

Baldissero (Samples 1 to 7). The samplesexamined come from the town of Baldissero,located in the subalpine area of Canavese; in thisarea, white, yellowish- and brownish-red opals,sometimes with dendrites of pyrolusite, arefound (Hintze, 1915). Also other kinds of opals,with a body color from blue to bluish-grey- andseveral arborescent inclusions of todorokite,were found in Bettolino, Baldissero and otherlocalities of Canavese (Mariani and Scaini,

F. Caucia et al.94 Periodico di Mineralogia (2012), 81, 1, 93-106


1978). In particular, opals with inclusions oftodorokite, such as those investigated in thiswork, are highly prized by collectors (Figure 1a,b).

Caselette (Samples 8 and 9). The samples examined originate from the town

of Caselette in the Susa Valley; fine specimens ofopals are frequently found here, usually inmagnesite deposits (Jerwis, 1873; Fenoglio andSanero, 1943). In particular, opal is still quite

abundant, sometimes with samples of appreciablevalue, in the mine of Mount Calvo in Casellette.The transparency of these opals varies fromtransparent to semi-opaque and the color fromwhite to brown. Besides Mount Calvo, in the SusaValley opals have been also found in Val dellaTorre, in Givoletto and Bussoleno near CavaTignai (Jerwis, 1873; Fenoglio and Sanero, 1943).

Bellino in Varaita Valley (Sample 10). The small town of Bellino, in the province of

Cuneo in Piedmont, is located in Varaita Valley;opals are present, in particular in the manganesedeposits on the Mount Maniglia, located at highaltitude (ca 3000 m; Jerwis, 1873).

Case Montanini (Sample 11). This locality is near Sant’Andrea Bagni

(Medesano) in the Province of Parma. Near CaseMontanini and in nearby Faieto, there are somequarries with interesting samples of opals: theygenerally occur as light blue in small veins orquite big spherules and semi-spheres, having awhite to intense light blue color, displaying asericeous (perly) luster and a moderateiridescence (Adorni et al., 2004).

Silius (Sample 12). The analyzed sample is ahyalite from a mine of barite and fluorite locatednear the village of Silius, in the province ofCagliari in Sardinia. This mine is one of the fewstill active in Italy. Opals from this site arecolorless to milky hyalite with a botryoidalsurface and, in some cases, also a considerablethickness. It usually occurs as coatings orincrustations but also nodules, especially inigneous rocks (Figure 2).

Materials and methods

The specimens were examined by standardgemological methods to determine their opticalproperties, hydrostatic specific gravity (SG),ultra-violet fluorescence (UV) and microscopicfeatures.

Specific gravity and refractive index (RI)measures of opal samples have been carried out

Physical and chemical properties of some italian ... 95Periodico di Mineralogia (2012), 81, 1, 93-106

Figure 1. Baldissero Samples: a) Sample 4, opal-CTwith a white color. b) Sample 6, ; opal- CT with ayellowish-white color and dendrites.

(a)

(b)


using a Presidium PCS100 Sensible Balance anda Kruss Rifractometer ER6040 equipment,respectively. Detection limits were 1.30 < n <1.80.

X-Ray Powder diffraction data (XRPD) onsamples of opals and the matrix that includedthem, have been collected with a Philips PW1800Powder Diffractometer, with CuKα radiation (λ = 1.5418 Å) and a scan speed of 1°/min, in therange between 2-65° 2θ.

Mid-infrared spectra (4000-400 cm-1) havebeen recorded in transmission mode using aNicolet Nexus FTIR spectrometer, equipped witha 4x beam condenser collector, accumulating 200scans at a resolution of 4 cm-1. We operated bymeans of KBr compressed pellets, after a pre-treatment of 150 °C and fluxing the samplecompartment with gaseous nitrogen.

LA-ICP-MS microanalyses were performedwith a double-focusing sector - field ICP-SFMSmodel Element I, ThermoFinnigan Mat at IGG-CNR of Pavia. Quantification was performedusing SiO2 (stoichiometric value) as internalstandard and NIST SRM 610 synthetic glass asexternal standard. Precision and accuracy wereestimated by the analysis of a BCR-2 standard(accuracy: 2 sigma), for concentration at ppmlevel. Opal fragments were mounted on epoxyresin and polished before analyses. Detection

limits for each element can be found in Miller etal. (2012).

Gemological properties

The appearance and gemological properties ofthe investigated Italian opals are reported in Table1. The opals span a range from colorless, white,brownish-white, yellowish-white, yellowish togreenish white to grayish; four samples are cut incabochon, and were chosen based on the differentkind of color shade; no sample shows a uniformcoloration. Luster is described as Vitreous (V) orGreasy (G) and the degree of transparency isdefined as translucent (TL) or opaque (O).Investigated opals appear vitreous or greasy, whilemost of them are generally translucent and onlytwo samples (samples 3 and 5 from Baldissero)are opaque. No samples are perfectly transparent.Samples 1, 2, 3 and 6 (from Baldissero) show thepresence of black to dark brown branched andopaque dendrites. Dendrites in opals 2 and 3 arein lower quantity. All samples are common opals,that is, none of them display a play-of-color; onlyfour opals from Baldissero exhibit smalliridescence zones (see Table 1). All the samplesare inert to long and short wavelength UVradiation (366 - 254 nm respectively), with theexception of the sample of hyalite from Sardinia(n. 12) that shows a typical bright yellowish-greenfluorescence that might be attributed to smallamounts of uranium, present as uranyl UO2(Fritsch et al., 2002; Olmi et al., 2002; Gamboniet al., 1997; 2004; Gaillou et al., 2012).Phosphorescence was never observed.Microscopic observations (observed with amagnification up to 40x) in transmitted andreflected light highlighted the presence of milkyzones, gas microbubbles, microfractures,dendrites and iron oxides.

Specific gravities and refractive indices

The average values of measured specificgravity and refractive indices of samples 1, 2, 9


Figure 2. Sample 12 from Silius; colorless hyaliteopal.


and 11 are reported in Table 2. The specificgravity of opals might be influenced by manyfactors such as the presence of microcracks,porosity or water content. For this reason, thegravity values have been evaluated only on themost appropriate samples that appeared compact,homogeneous with no presence of empty orrecrystallised fractures. In general, the values ofspecific gravity of the four investigated samplesare rather high but in the range of data inliterature (2.0-2.25; Klein, 2004). Sample 1shows higher values most likely due to thepresence of dendrites. Similarly, refractive

indexes are comparable with the literature data(around 1.39-1.48; O’Donaghue, 2006; Caucia etal., 2009; Simoni et al., 2010).

X-Ray Powder Diffraction (XRPD) on opals

XRPD investigations allow distinguishing theopals into three general groups (Jones andSegnit, 1971; Ghisoli et al., 2010): opal-C(relatively well ordered α-cristobalite), opal-CT(disordered α-cristobalite with α-tridymite-typestacking) and opal-A (amorphous). In the XRPDpatterns of the investigated opals, the crystallinepart, made up by only cristobalite or by amixture of cristobalite and tridymite, isgenerally mixed with the typical amorphouscomponent of the opal (Figure 3 A, B C). Thecristobalite versus tridymite ratio was calculatedby comparing the position of the characteristicpeak in the interval d = 4.04-4.11 Å, whoseextremes correspond respectively to the valuesof pure cristobalite and pure tridymite. Inparticular, opal-CT shows a peak correspondingto a d value ranging from 4.06 to 4.11 Å, while


Table 1. Gemological properties of the studied samples.

Sample Origin Description Color Transparence Luster Optical Type effects

1 Baldissero rough/cabochon Yellowish-white TL V iridescence CT (close to with dendrites tridymite) 2 Baldissero rough/cabochon greyish TL G iridescence CT 3 Baldissero rough Yellowish- to O G - CT greenish- white 4 Baldissero rough white TL V - CT 5 Baldissero rough brownish white O G - CT 6 Baldissero rough yellowish white TL V iridescence CT with dendrites 7 Baldissero rough greyish TL V iridescence pure C 8 Caselette rough yellowish white TL V - CT 9 Caselette rough/cabochon brownish white TL G - CT 10 Bellino rough yellowish white TL G - A 11 Case rough/cabochon white TL V CT Montanini 12 Silius rough colorless TL V - A

TL = translucent, O = opaque, V = Vitreous, G = Greasy.

Table 2. Physical and optical characteristics of thestudied gems.

Sample Cut Weight Specific Refractive gravity index

1 cabochon 0.076 g 2.33 g/cm3 1.442 2 cabochon 0.752 g 2.07 g/cm3 1.440 9 cabochon 0.096 g 2.07 g/cm3 1.430 11 cabochon 0.078 g 2.07 g/cm3 1.430


opal C is characterized by a peak between 4.02and 4.05 Å and by the presence of two otherpeaks at 3.13 and 2.84 Å (Fritsch et al., 2004:Ghisoli et al., 2010). For the evaluation of theratio, we used the following equation (Ghisoliet al., 2010): d Trid - d measured 4.11 Å - xC/TXRPD = = d Trid - d Crist 4.11 Å - 4.04 Å

XRPD analyses showed that samples 10 and12 are fully amorphous; sample 1 is a CT typeclose to pure tridymite (C < 10 % T > 90 %)while sample 7 is a “pure” C type as is almostcompletely made up by well crystallized α-cristobalite. Opals 8, 9 (from Caselette), 2, 3, 4,5, 6 (from Baldissero) and 11 (Case Montanini)belong to the CT type with a crystalline portionvariably composed by C = 43% T = 57%(samples 2 and 9), C = 14% T = 86% (samples 3and 4), C = 28 % T = 72 % (samples 5 and 8),and C = 71 % T = 29 % (sample 6). In Figure 1A, B and C, some typical XRD patterns of opals-CT, -A and -C s investigated in this work arereported. In general, opals-C and -CT are ofvolcanic origin, opals-C being rarer than opals-CT (Jones and Segnit, 1971; Ostrooumov et al.,1999; Fritsch et al., 2004; Simoni et al., 2010).Our results are also consistent with the findingsby Elzea and Rice (1996) that opal-C and opal-CT are part of a continuum of disorderedintergrowths between end-members cristobaliteand tridymite stacking sequences.

X-ray Powder Diffraction (XRPD) on matrix

We analysed two samples of matrix fromCaselette, one from Baldissero and one fromCase Montanini. Unfortunately, since thematerial that was possible to collect was veryfew, these analyses cannot provide completeindication about the mineralogical compositionof the host rocks. Anyway they can be veryuseful to determine the provenance of some trace

elements in the opals. The matrix of opals from Caselette is mostly

made up by abundant cristobalite, tridymite andsmectite, with few corrensite.

The matrix of one opal from Baldissero ismostly made up by tridymite, lower cristobalite,some todorokite and clay minerals likechlorite/smectite mixed layers. The presence ofclay minerals indicate the source rocks underwentextensive alteration, probably due to weatheringor hydrothermal processes, that might also berelated to the formation of opals.

The matrix of the samples from Case Montaniniis mostly made up by Mg Calcite with loweramount of feldspar, chlorite, muscovite, quartzand dolomite.

IR Spectroscopy

We have analyzed through IR spectroscopy thesamples n. 1, 3, 9 and 12 (Figure 4A, B, C, D)and the results have been compared with theXRPD data; the combination of these analysescan provide useful information about thecomposition of the opals. By XRPD, the opals 1,3, 9 and 12 have been identified as follows :sample n. 1 trydimitic opal-CT (C < 10 %, T >90 %); samples n. 3 and 9 opal-CT, sample n. 12A-opal (Table 1).

The samples investigated in this work arecharacterized by an absorption band in the range3435 - 3467 cm-1 due to the OH stretchingvibration of water molecules as well as the waterbending vibration at ~ 1638 cm-1. We alsoobserve three other strong bands at ~ 1105 cm-1,790 cm-1 and 475 cm-1 that are common to allsilicates with tetrahedrally coordinated siliconand are generally assigned to the stretchingvibration of Si-O.

In particular, the 1100 and 790 cm-1 bands aregenerally assigned, respectively, to antisymmetricand symmetric Si-O-Si stretching, whereas the470 cm-1 band is related to O-Si-O bendingvibration (Jones and Segnit, 1971; Farmer, 1974;




Figure 3. XRPDpatterns of theinvestigated opals. A) sample n. 7(from Baldissero),opal-C; B) sample n. 9(from Caselette),opal-CT; C) sample n. 12(from Silius),opal-A.

A

B

C


Fritsch et al., 2004; Brajkovic et al., 2007; Cauciaet al., 2008; Adamo et al., 2010).

In general C-opals show the absorption bandsof well-ordered α-cristobalite, consisting of apeak at about 620 cm-1 and a shoulder at 1198-1200 cm-1 on the 1100 cm-1 absorption band.These absorption bands, in particular the 620cm-1 sharp peak, are always absent in the FTIRspectra of opal-CT and A, and are thereforediagnostic markers for opal-C (Jones and Segnit,1971; Farmer, 1974; Caucia et al., 2009; Adamoet al., 2010). Since these bands are not present,our samples can’t be classified as C opals.

In general, the band at 790 cm-1 shows thehighest value in opal-A (796-801 cm-1), thelowest in opal-CT (788-792 cm-1), whereas opal-C has intermediate value (793-794 cm-1; Adamoet al., 2010). The samples n. 1 (Figure 4A), 3(Figure 4B) and 9 (Figure 4C) show a band atabout 789-1 and can therefore be considered as CT-opals while the sample n. 12 (Figure 4D) showsthe band at 801-1, which is typical of A-opal.

In conclusion, IR spectroscopy highlighted thatthe opals 1, 3, 9 (from Piedmont) can be identifiedas CT-opals, while the opal n. 12 (from Silius) isan A-opal. The tridymitic opal CT (sample n. 1)cannot be distinguished from the other opals CT.IR spectroscopy analyses therefore confirm theXRPD results.

Laser Ablation ICP-MS Analysis

The opal samples 1, 6, 8, 9 and 12 have been

analyzed through LA-ICP-MS microanalyses forthe determination of impurities and trace elementscomposition, with particular attention to thechromophore elements (Table 3).

It is known that the chemical composition ofopals is mostly linked to that of host rocks andinfluences opal properties like color andluminescence (McOrist and Smallwood, 1997;Gaillou et al., 2008; Caucia et al., 2009; Simoniet al., 2010). The body color is usually related toinclusions of colored minerals while theabundances of some elements determine the colorof the opal (McOrist and Smallwood, 1997;Fritsch et al., 2004; Gaillou et al., 2008, andreferences therein). To the best of our knowledge,analyses on the chemical composition of traceelements in the Italian opals have never beenconducted before. Therefore, in this work, therelationships between the color and chemicalcomposition of some Italian opals have beeninvestigated for the first time. The main detectedimpurities resulted to be Mg (400-900 ppm), Fe(35-420 ppm) and, subordinately, Ca (72-111ppm) and Ni (20-65 ppm). Mn was clearlydetected (about 40 ppm) in the samples 1 and 6while is absent in the others. Sc and Cr have beendetected in all the samples, but in very lowamount (4-6 ppm and 4-11 ppm respectively). Bawas detected, in low amount, only in the samples1 and 6 while is absent in the others. Elementslike Ti, V, Co, Pb, Zn, are present onlyoccasionally and in low amounts. REE, U, Rb, Y,Zr, Nb, Cs, Hf, Au, Pb, Sn, Th, As, Ag resulted


Table 3. Trace elements abundances (ppm) for the opals analysed in this work. Columns in bold includechromophore elements.

Samples Mg Fe Ca Ni Cr Sc Ba Ti V Co Pb Zn Mn Cu Li

1 900 250 111 56 7 6 5 2 1 1 10 6 42 3 - 6 880 268 72 52 6 6 4 1 1 1 8 6 43 3 1 8 760 400 96 65 10 5 - 5 2 - 8 2 - 15 - 9 700 421 75 51 10 6 - 5 2 - - 1 1 28 - 12 400 35 92 20 4 4 - - - - - - - - -



Figu

re 4

. IR

spec

tra o

f the

ana

lyze

d op

als.

A) s

ampl

e 1

from

Bal

diss

ero

Can

aves

e (C

T op

al);

B) s

ampl

e 3

from

Bal

diss

ero

Can

aves

e (C

T op

al);

C)

sam

ple

9 fr

om C

asel

ette

(CT

opal

); D

) sam

ple

12 fr

om S

ilius

(A o

pal).

AB

CD

1 9123


below the detection limits. Samples 1 and 6 from Baldissero are

distinguished by the highest Mg content (around900 ppm), the appreciable amount of Mn (around40 ppm) and some Pb, Ba and Zn. Samples 8 and9 from Caselette differ from the others for thehighest Fe content (400 and 421 ppm). Sample12, which is a hyalite opal from Silius (opal Awith botroydal aspect), is characterized by thelowest Mg content (400 ppm), Ni (20 ppm) andespecially Fe (35 ppm). In general, we observethat typical chromophore elements like V, Cr, Cu,Ti, Co (columns in grey) are low in all theinvestigated opals and their presence does notseem related to the color of the gem. Ni is moreabundant and can be responsible for the yellowishgreen color, but no UV -visible absorptionspectrum has been acquired to confirm thishypothesis. Conversely, Fe contents are likelyrelated with the color of the opals: the brownishcolor of sample 9 is due to the highest content ofFe while the lower quantities in opals 1, 6 and 8determine the yellow tint. Sample 12, whichfeatures the lowest Fe content, is the onlycolorless opal. A clear correlation between Fecontent with the color of the gem was alsoobserved in the opals from Bemia in Madagascar(Table 4) (Simoni et al., 2010), from Brazil(Caucia et al., 2009) (Table 5) and otheroccurrences as well (Gaillou et al., 2008). Fe inopals probably occurs as inclusions of Fe

oxyhydroxides that determine the yellow ororange color (Fritsch et al., 2002, Gaillou et al.,2008). Mn has been detected only in samples 1and 6 and is due to the presence of dendrites oftodorokite; as a matter of fact this mineral hasbeen also detected by XRD investigation on thematrix. Ca and Mg are non-chromophoreelements, and are present in relevant amounts; Mgprobably derives from the matrix, which is largelyformed by Mg rich minerals like smectite,smectite/chlorite mixed layer and corrensite (seealso Jerwis 1873; Fenoglio and Sanero 1943).Also, we observe that the Italian opalsinvestigated in this work are characterized by verylow amounts of Ba, that are around 5 % or alsobelow the detection limits. According withGaillou et al. (2008), Ba concentration allow todifferentiate sedimentary opals from volcanicones, as the first feature Ba contents > 120 ppm,while the volcanic opals show lower contents.This is because Ba in the sedimentary opalsgenerally derives from the dissolution, throughseveral weathering cycles, of the detrital feldsparsoccurring in sedimentary rocks, that issubsequently dissolved into the water from whichthe opals form. Conversely, feldspars in avolcanic environment are subjected to only onecycle of weathering and, therefore, the Ba contentin water is much lower (Fagel et al., 1999;Davidson et al., 2005; Gaillou et al., 2008). Thevery low Ba values of Italian opals support a


Table 4. Trace elements abundances (ppm) for opals from Bemia, Madagascar (from Simoni et al., 2010).

Samples Mg Fe Ca Ni Cr Sc Ti V Co Pb Zn Mn Cu Li color <> <> <> <> <> <> <> <> <> <> <> <> <> <>

yellowish 1122 137 1238 1 1 3 54 1 1 1 1 2 1 1 white yellow 891 1888 370 5 4 5 62 4 1 1 7 13 2 1 orange 17730 12070 1314 3 6 3 12 2 0.5 6 87 10 2 1

<> = Average value


volcanic origin; this is also demonstrated if weplot our results in the Ca-Ba diagram fromGaillou et al. (2008), that reports the differentcompositional fields of opals in the world.

It can be interesting to compare the trace elementcomposition of the opals analyzed in this workwith that of blue and fire opals from Piauí in Brazil(Caucia et al., 2009; Table 5); fire opals fromBemia in Madagascar (Simoni et al., 2010; Table4) and other occurrences worldwide (Gaillou et al.,2008), to evaluate differences and similarities.

The most surprising result in our samples isrepresented by the relatively high content of Ni,as this element is generally present in muchlower amount in the opals worldwide (frequentlyless than one ppm; Gaillou et al., 2008). Largequantities of Ni have been detected only in thegreen opals from Tanzania and in those fromSilesia, where Ni is contained in the structure ofpimelite, a Ni rich variety of smectite (Schmetzeret al., 1976; Koivula and Fryer, 1984; Shingleyet al., 2009). It is likely that also Ni in oursamples is contained in the structure of clayminerals like smectite or mixed layers, thateasily incorporate or adsorb transition elements;these mineral have been indeed detected in thesamples of matrix from Caselette and Baldissero.Even if the number of analysed samples is quitelow, Ni abundance might be therefore used asmarker to characterize the Italian opals(especially from Piedmont).

According to Gaillou et al. (2008), Ca is theonly element in opals that varies significantly

with the geographic origin of the samples and cantherefore be useful to determine the provenance.Ca contents in Italian opals are quite low but inthe range of the opals worldwide and show ahomogeneous composition. Ca contents can becompared with that of opals from Mexico andHonduras (Gaillou et al., 2008) but are lower, ormuch lower, of that of opals from other regionswhere this element show concentration of somethousands ppm (for instance, Madagascar; Table4; see also Gaillou et al., 2008). Concerning Mgand Fe, these elements in opals worldwide canshow a large variability, between few units tosome thousands ppm, but cannot be related tosome specific areas (see Gaillou et al., 2008;Tables 4 and 5). Mg and Fe contents in Italianopals are in the range of opals worldwide andshow a homogeneous composition. For instance,we observe that Mg contents in Italian opals arehigher than in the opals from Brazil but lowerthan those from Madagascar (see Tables 4 and 5).Also, Fe in Italian opals is higher than in the skyblue opals, and much lower than the brick redopals from Brazil or the yellow and orange fromMadagascar. Lastly, opals from the mentionedforeign localities exhibit a variability larger thanthat observed in the Italian samples, whichappears more limited. Even if the number ofsamples analyzed in this work is rather limited, itappears that the trace element composition ofItalian opals is rather homogeneous, especially inthe case of samples of common opal fromPiedmont. Sample n. 12, which is the hyalite opal


Table 5. Trace elements abundances (ppm) for opals from Piauì, Brazil (from Caucia et al., 2009).

Samples Mg Fe Ni Cr Sc Ti V Co Mn Cu Ba color <> <> <> <> <> <> <> <> <> <> <>

sky blue 10 3 0.5 0.5 4 2 1 1 0.5 9 4gold yellow 49 132 1 1 4 16 0.5 - 0.5 13 - brick red 60 1164 1 1.5 7 86 0.5 0.3 7 29 15

<> = Average value


from Sardinia, partly differs from the others forthe even lower content in trace elements.

Conclusions

Italian opals do not have the relevantgemological properties, such as the play-of-color, typical of some opals from other countries.However, some samples of Italian opal areaesthetically valuable for their qualities such asiridescence, the presence of metallic inclusionsof todorokite and their color variability. As aresult, and also for their rarity, Italian opalsappear to be well appreciated by collectors.Italian opals analyzed in this work show differentcolors and variable degrees of luster andtransparency; black and metallic inclusions,consisting of todorokite, are often observed. Noplay-of-colors have been observed, but someopals show the presence of iridescent zones;play-of-colors, as well as iridescence, areappreciated optic qualities in the gem market.Several samples show a typical opalescence andthe fracture is usually conchoidal. The values ofspecific gravity and refractive index of four cutopals are comparable with the range of data inliterature. XRPD and IR spectroscopy analyseshighlighted Italian opals can be classified as A,CT and C types, but most of them are opals-CT.This composition supports a volcanic origin. Theanalyses on the matrix of opals from Piedmontalso highlighted the occurrence of cristobalite,tridymite, todorokite, clay minerals like smectite,corrensite and chlorite/smectite mixed layers.The presence of clay minerals indicates thesource rocks underwent extensive alteration,probably due to weathering or hydrothermalprocesses; these processes might also be relatedto the formation of opals.

Impurities and trace element contents in theopals can be, at least partially, related to the colorand composition of the matrix of the opal. Themost abundant impurities are Mg, Fe, Ca and Ni.Ca and Mg (non chromophore elements) are

probably inherited from the matrix; in particularMg can derive from clay minerals. Ba showsvery low content and this would support avolcanic origin of the opals. Chromophoreelements like V, Cr, Cu, Ti, Co are present invery low contents and do not seem to influencethe physical properties of the Italian opalsanalyzed. Ni is present in noticeable amounts,might lead to a yellowish-green color of asample, but does not seem responsible for thecoloration of the others; it probably derives fromclay minerals. Mn is clearly detected only in twosamples and is related to the presence ofdendrites of todorokite. As also observed inseveral opals from other parts of the world, Feappears to be the main factor that determines thered to brown component. Although the numberof analysis performed in this work is limited, itseems that the Italian opals show a ratherhomogeneous trace element composition whichis in the range of composition for opals fromother locality, except for their unusual Nicontent. Ni could therefore be a usefulgeochemical tracer to separate Italian opals fromopals from other localities.

Acknowledgements

The authors are grateful to Dr. Alberto Zanetti (IGG-CNR - Pavia) for the assistance with LA-ICP MSanalysis. Special thanks also to the “AssociazioneMicro-mineralogica Italiana”, to “GruppoMineralogico Basso Canavese” and “Museo DonBosco di Torino”, that kindly provided the samples forthe analysis. We are also particularly grateful to EloiseGaillou and an anonymous referee for reviews andimprovement of the manuscript.

References

Adamo I., Ghisoli C. and Caucia F. (2010) - Acontribution to the study of FTIR spectra of opals.Neues Jarbuch für Mineralogie Abhandlungen, 187,63-68.

Adorni F., Tadeo F. and Adorni B. (2004) - Lamelanoflogite di Case Montanini. Rivista



Mineralogica Italiana, 3, 126-136. Altaner S.P., Ylagan R.F., Savin S.M., Aronson J.L.,

Belkin H. and Pozzuoli A. (2003) - Geothermometry,geochronology and mass transfer associated withhydrothermal alteration of a rhyolitic hyaloclastitefrom Ponza Island, Italy. Geochimica etCosmochimica Acta, 67, 275-288.

Brajkovic A., Rolandi V., Vignola P. and Grizzetti R.(2007) - Blue and Pink Opals from Acari, Peru -Their optical, structural and spectroscopic features.The Australian Gemmologist, 23, 1, 3-15.

Caucia F., Ghisoli C., Adamo I. and Bocchio R. (2008)- Opals-C, opals-CT and opals-T from Acari, Peru:X-ray powder diffraction analysis and IRspectroscopic investigation of new samples showingtwo different typologies of lustre. The AustralianGemmologist, 23, 266-271.

Caucia F., Ghisoli C. and Adamo I. (2009) - A study onthe characteristics of some C- and CT-opals fromBrazil. Neues Jarbuch fur MineralogieAbhandlungen, 185, 289-296.

Cerri G., Cappelletti P., Langella A. and de Gennaro M.(2001) - Zeolitization of Oligo-Miocenevolcaniclastic rocks from Logudoro (northernSardinia, Italy). Contribution of Mineralogy andPetrology, 140, 404-421.

Davidson P., Kamenetsky V., Cooke D. and Frikken P.(2005) - Magmatic Precursors of HydrothermalFluids at the Río Blanco Cu-Mo Deposit, Chile:Links to Silicate Magmas and Metal Transport.Economic Geology, 100, 963-978.

Elzea J.M. and Rice S.B. (1996) - TEM and X-raydiffraction evidence for cristobalite and tridymitestacking sequences in opal. Clays & Clay Minerals,44, 492-500.

Fagel N., André L. and Dehairs F. (1999) - Excess Baadvective transport evidenced from trap and sedimentgeochemical signatures. Geochimica etCosmochimica Acta, 63, 2353-2367.

Farmer V.C. (1974) - The infrared spectra of minerals.Mineralogical Society, London, UK, 539 p.

Fenoglio M. and Sanero E. (1943) - Sulla presenza ediffusione della cristobalite negli opali dei giacimentidi magnesite delle Prealpi Piemontesi. AttiAccademia delle Scienze, Torino, Cl. Sci. Fis., Mat.Nat. 78, 265-273.

Fritsch E., Ostrooumov M., Rondeau B., Barreau A.,Albertini D., Marie A.M., Lasnier B. and Wery J.(2002) - Mexican gem opal: nano- and micro-

structure, origin of color and comparison with othercommon opals of gemmological significance. TheAustralian Gemmologist, 21, 230- 233.

Fritsch E., Gaillou E., Ostroumov M., Rondeau B.,Devouard B. and Barreau A. (2004) - Relationshipbetween nanostructure and optical absorption infibrous pink opals from Mexico and Peru. EuropeanJournal of Mineralogy, 16, 743-752.

Gaillou E., Delaunay A., Rondeau B., Bouhnik Le CozM., Fritsch E., Cornen G. and Monnier C. (2008) -The geochemistry of gem opals as evidence of theirorigin. Ore Geology Reviews, 34, 127-133.

Gaillou E., Fritsch E. and Massuyeau F. (2012) -Luminescence of gem opals: a review of intrinsic andextrinsic emission. Australian Gemmologist, 24, 200-201.

Gamboni A., Gamboni T. and Nonnis O. (1997) - Iminerali dell’arcipelago di La Maddalena - P. SorbaEditore, La Maddalena.

Gamboni A., Gentile P., Mascia S., De Vito A. andGamboni T. (2004) - Monte Canu: uranofane-beta ealtri minerali delle pegmatiti granitiche. RivistaMineralogica Italiana, 3, 138-145.

Ghiara, M.R., Petti C., Franco E., Lonis R., Luxoro S.and Gnazzo L. (1999) - Occurrence of clinoptiloliteand mordenite in Tertiary calc-alkaline pyroclastitesfrom Sardinia (Italy). Clays & Clay Minerals, 47,319-328.

Ghisoli C., Caucia F. and Marinoni L. (2010) - XRPDpatterns of opals: A brief review and new results fromrecent studies. Powder Diffraction, 25, 274-282.

Giuliani G., Chaudisson M., Schubnel H.J., Piat D.,Rollion Bard C., France Lanord C., Giard C., DeNarvaez D. and Rondeau B. (2000) - Oxygenisotopes and emerald trade routes since antiquity.Science, 287, 631-633.

Hintze C. (1915) - Handbuch der Mineralogie. I (2).Leipzig, Verlag Von Veit.

Jerwis G. (1873/1881) - I tesori sotterranei d’Italia. Ed.Loescher, Torino.

Jones J.B. and Segnit E.R. (1971) - The nature of opal.Part 1: Nomenclature and constituent phases. Journalof the Geological Society Australia, 8, 57-68.

Klein C. (2004) - Mineralogia. Zanichelli Bologna. Koivula J.I. and Fryer, C.W. (1984) - Green opal from

East Africa. Gems & Gemology, 20, 226-227.Mariani P. and Scaini G. (1978) - I minerali d’Italia.

Rizzoli. Mc Orist G.D. and Smallwood A. (1997) - Trace



elements in precious and common opals usingneutron activation analysis. Journal ofRadioanalytical and nuclear Chemistry, 223, 9-15.

Miller C., Zanetti A., Thöni M., Konzett J. and KlötzliU. (2012) - Mafic and silica-rich glasses in mantlexenoliths from Wau-en-Namus, Libya: Textural andgeochemical evidence for peridotite-melt reactions.Lithos, 128, 11-26

O’Donaghue M. (2006) - Gems, 6th ed. Butterworth-Heinemann, Oxford, UK.

Olmi F., Sabelli C., Senesi F. and Stara P. (2002) - Lamineralizzazione uranifera di Arcu su Linnarbu. -Rivista Mineralogica Italiana, 3, 148-169.

Ostrooumov M., Fritsch E., Lasnier B. and Lefrant S.(1999) - Spectres Raman des opales: Aspectdiagnostique et aide a la classification. EuropeanJournal of Mineralogy, 11, 899-908.

Schmetzer K., Berdesinski W. and Krupp H. (1976) -Grüner Opal aus Tansania. Der Aufschluß, 27, 381-384.

Van Der Marel H.W. and Beutelspacher H. (1976) -Atlas of infrared spectroscopy of clay minerals andtheir admixtures. Elsevier Scientific PublishingCompany, Amsterdam.

Shigley J.E., Laurs, B.M. and Renfro N.D. (2009) -Chrysoprase and Prase Opal from Haneti, CentralTanzania. Gems & Gemology, 45, 271-279.

Simoni M., Caucia F., Adamo I. and Galinetto P. (2010)- New occurrence of fire opal from Bemia,Madagascar. Gems & Gemology, 46, 2, 114-121.

Submitted, June 2011 - Accepted, March 2012



Documents

Physical and chemical properties of some italian opals - Periodico di